Basic Concepts – Conditionals Conditionals are used for making decisions. The conditionals available in C are if and if-else statements the switch statement the conditional expression operator, ?:

Operators – Relational The following relational operators are available in C: == equality != not equal < lesser than > greater than <= less than or equal to >= greater than or equal to

Conditionals – if The basic format of the if statement is if (condition_is_true) do_something; Examples if (x >= 2) y = 10; if (y == 10) printf("y is now 10");

Conditionals – if What if we wish to have a second conditional statement in response to the first condition being true? We can nest the if statements. Example if (count > 23) if(time < 45) z = 56; This assigns a value of 56 to z only if count > 23 and time < 45.

Conditionals – if cont. Question: What if we want to do more than one thing in response to a condition being true? Answer: Create a block of statements. Example if (x >= 2) { y = 10; printf("y is now 10"); }

Conditionals – if-else Sometimes we wish to do one thing if a condition is true but another if the condition is false. For this we can use an if-else statement: if (condition_is_true) do_something; else do_something_else; Example if (x >= 2) printf("x is greater than or equal to 2"); else printf("x is less than 2");

Conditionals – if-else What if we have nested if statements and an else statement; which if does the else belong to? /* indented to trick you */ if (amount == 13) if (cost == 52) printf("cost is %d\n", cost); else printf("got here\n"); /* indented to show correct logic */ if (amount == 13) if (cost == 52) printf("cost is %d\n", cost); else printf("got here\n");

Conditionals Notes Keep in mind that any condition that evaluates to a nonzero value is considered true. if (8) printf("non-zero values are true\n"); else printf("this never prints\n"); /* another example */ if (-3.4) printf("-3.4 is considered true\n"); else printf("this never prints\n"); /* another example */ if (0) printf("zero is false\n"); else printf("this is always false\n");

Conditionals Notes cont. WARNING: Don’t use = when you really mean == = is used for assigning values Example: a = 5; == is used for determining if two values are equal Example: if (a == 5)

Conditionals – switch An if-else statement is used for binary decisions–those with two choices. Sometimes there are more than two choices. switch (expression) { case label1: do_this case label2: do_this · case labeln: do_this default: do_this }

Conditionals – switch cont. Notes on the use of the switch statement The labels must be integers (or at least evaluate to an integer). The default line is optional. Once the matching label is found, that statement and each following statement will be executed (unless we use a break).

switch Example Example: #include int main( void ) { int a = 3; int b = 5; switch(b - a) { case 5: printf("countdown from 5\n"); case 4: printf("countdown from 4\n"); case 3: printf("countdown from 3\n"); case 2: printf("countdown from 2\n"); case 1: printf("countdown from 1\n"); }

switch Example cont. Output countdown from 2 countdown from 1

switch and break On many occasions we may only wish to do a single thing in response to what the switch evaluates to. For this, we can use a break statement. Example: #include int main(void) { int cost = 18; switch(cost) { case 23: printf("cost is 23\n"); break; case 18: printf("cost is 18\n"); break; case 75: printf("cost is 75\n"); break; }

switch and break contd. Output cost is 18

Conditionals – ?: Instead of an if-else statement, we can use the conditional expression operator: (condition_is_true) ? do_if_true : do_if_false; Example (w < 14) ? (x = 10) : (y = 19); NOTE: This is discussed in chapter 9 (page 515) of the textbook.

Conditionals – ?: cont. A difference between this and an if-else statement is that the conditional expression can be used like other expressions: Example answer = ((w > 14) ? 28 : 16); This is equivalent to if (w > 14) answer = 28; else answer = 16;

Conditionals Example 1 Both types of conditionals are used equivalently here #include int main(void) { int x = 3, y = 10; printf("x is %d, y is %d\n\n", x, y); if (x > y) printf("x is larger\n\n"); else printf("y is larger\n\n"); /* equivalent structure */ printf("Let’s try with the conditional expression:\n"); (x > y) ? printf("x is larger\n") : printf("y is larger\n") ; }

Conditionals Example 1 cont. The output is x is 3, y is 10 y is larger Let’s try with the conditional expression: y is larger

Conditionals Example 2 Here we have a conditional expression inside a function call: #include int main(void) { int x = 5; printf("x is %s\n", (x < 100) ? "small" : "large"); } Produces x is small

Increment/Decrement Operators The following operators are available in C for incrementing and decrementing variables by a value of one: ++ increment -- decrement Example: We could use a = a + 1; or a++;

Increment/Decrement Operators These operators can be placed before (prefix) or after (postfix) a variable: int x = 5; x--; /* at this point x has a value of 4 */ Or int x = 5; --x; /* at this point x has a value of 4 */ both reduce x by one. Later we will see examples where the choice of prefix or postfix matters.